When I execute the scpg3 from the command prompt to copy one of the files, I get:
scpg3: warning: Write failed: No process is on the other end of the pipe.
The particular file involved was 264 KB (270,731 bytes)

Version 5.1.x of client had great many issues. The product was getting more and more stable with every release. I do not recommend you to troubleshoot your current configuration at all. Moreover, both: your client version and server versions are no longer supported.

In addition, the 6.1.4 version of server contains a serious vulnerability (there is a workaround and fix in 6.1.13). Details here.

So I would recommend to upgrade to latest version (long term supported is 6.3.10; development, but stable version is 6.4.6). And if the problem persists, try to ask here again.

Few hints for troubleshooting 5.1.x: some issues in earlier versions of clients were related to implementation of proprietary extension of SFTP protocol called streaming and a "checksum feature". Therefore, you can try to experiment with 2 options of scpg3:

and similar. You could also try to make sure that the file at destination is not present before copy. There were some issues related to overwriting files. But if you are hitting some very nasty bug, you may be out of luck with 5.1.x.

6.0 version started to be more usable. And since 6.2 it is quite stable. So I believe current versions would work just fine.